The geochemistry of manganese conglomerates on the Vityaz’ Rise, Japan Sea

Abstract

Large conglomerate fragments up to 25 cm across were dredged off the submarine volcanic Vityaz Rise, Japan Sea in the 1600–1350 m depth range. These are the first ever that were recovered in this location. The fragments consist of pebble and gravel material that is cemented by manganese hydroxides. The cement is basal in type; the chemical composition is M2+MnO2, where M2+ = Na2, Mg, K2, Ca, Ba. The Mn/Fe ratio is 802. The upper part of the conglomerates is covered with a layered crust of the same hydroxides. A similar crust can be discerned within the conglomerate closer to the bottom of a sample. Some layers of the crusts are rich in NaCl, CaSO4, F, Co, Cu. The crusts contain inclusions of grains that consist of Ag and intermetallic compounds based on Cu and Ni. The superimposed manganese mineralization is Quaternary. The metals originated, most likely, from gas-rich hydrothermal fluids.